1. Field of the Invention
The invention is related to the field of radio communications, and in particular to radio communication systems employing time and frequency-divided multiple access schemes.
2. Related Technology
In any communication system including several users sharing the transmission medium, i.e. the available communication resources, special attention must be given to the co-existence of the different signals being present within the communication system. The users of the communication system generally share the same pool of communication resources. When allocating the communication resources (for example different channels) to the multiple users, it is realized that the signal of one user may affect (interfere with) the signal of another user. A communications system designer thus has to design a user traffic multiplexing scheme bearing this in mind, and thus design the multiplexing scheme so as to handle this undesired interference.
In communication systems in which a geographical division is used, i.e. a cellular system, there are mainly two kinds of multi-user interference present. Firstly, the interference from users within the same geographical area, called a cell, the so called intra-cell interference, and secondly the interference from users in adjacent (neighboring) cells, the so called inter-cell interference. The inter-cell interference may be decreased for example by means of resource planning (e.g. frequency planning), so that a specific communication resource is reused in such a way that interference is minimized. For frequency planning the inter-cell interference may be minimized by using a frequency reuse scheme, in which a certain frequency is not used in neighboring cells. However, resource planning, for example frequency planning and coordination between cells, is most time consuming, rather expensive and in some cases not even feasible.
Besides resource planning, whereby inter-cell interference may be decreased, there are other ways to decrease interference. One way to decrease both intra-cell interference and inter-cell interference is to utilize frequency hopping. Frequency hopping consists in changing the frequency used by a channel at regular intervals. Thus, cells using the same frequencies but different, presumably de-correlated, hopping sequences lead to decreased interference.
WO03/001696 describes a method for decreasing inter-cell interference. Frequencies are allocated to cells in a communication system according to functions selected to minimize repeated collisions between hopping sequences used by the base stations of neighboring cells. This is thus an example of a prior art method for decreasing inter-cell interference in a communication system, and in which system also resource planning is performed.
The international patent application PCT/CN03/00133 describes a method for interleaving in an OFDM system. The method described exploits the two-dimensional time-frequency structure of an OFDM system, and maps OFDM units onto frequency sub-bands in a way that maximizes the diversity performance.
In the prior art discussed above, the different aspects of allocating communication resources are treated more or less one at a time. It would however be desirable to provide a method for allocating communication resources to several users, in which all the aspects are considered simultaneously. A desired method should thus take into account the interference within a cell as well as the interference between cells in such a way that the total interference is minimized, whilst maximizing the diversity performance for each user. Further, it would be desirable to provide such method where expensive; time consuming and tedious resource planning is avoided.